1. Field of the Invention
The present invention relates to a spectrophotometer, and more particularly, to a spectrophotometer using a pulsed xenon flashtube.
2. Description of Related Art
A spectrophotometer is used to make measurements of the spectral reflectance of an object within a predetermined period of time and to perform a color matching operation. While the optical configuration of the spectrophotometer is capable of taking many forms, the like source thereof irradiates a sample and the light reflected from the sample is introduced into a spectroscope so that spectrum reflectance data can be obtained.
While the spectrophotometer uses various light sources therein, it normally uses a tungsten lamp. In order for the tungsten lamp to provide reliable spectrum reflectance data, it is necessary that the signal-to-noise ratio ((S/N) ratio) thereof be high which is effected by the emission of an intense light from the light source thereof. However, the tungsten lamp has a disadvantage in that the heat givent off thereby is great when the light emission intensity is high.
U.S. Pat. No. 4,076,421 or its corresponding Japanese Patent Publication No. 52183/1978 discloses a spectrophotometer using a pulsed xenon flashtube as the light source thereof. The xenon flashtube emits a small amount of heat and has a high S/N ratio compared with a tungsten tube. The reason why the pulsed xenon flashtube is used as the light source is because the spectral content thereof is extremely stable and enables a compensation for flash-to-flash variations using a single broadband-response photodetector. That is, the high intensity and short pulse width renders an electronic circuit which is high-pass filtered extremely insensitive to the effects of ambient light. Thus, the known spectrophotometer is intended to allow the light source thereof to emit an intense light. It is to be noted herein that 15 joules of energy is applied to spectrophotometers according to U.S. Pat. No. 4,076,421 and that the xenon flashtube provides a short intense pulse of illumination of approximately tens of microseconds duration.
However, when a spectral reflectance is determined using a light source which provides such an intense pulse of illumination by applying a great amount of energy to a spectrophotometer, the following occurs. Light may permeate deep into the sample to a great extent depending on the surface structure of the sample. Thus, even undesired information of the sample may be obtained. Data obtained using such a spectrophotometer has no correlation to the data obtained under natural light (or equivalents thereof). The intensity of light emitted from the light source of the spectrophotometer fluctuates to a great extent. Therefore, when the spectral reflectance of the sample having a high reflection factor is detected, it is difficult to compensate for the fluctuation of the intensity of the light reflected therefrom. Thus, accurate data of the spectral reflectance cannot be obtained.
In addition, the period of time required for charging the spectrophotometer, namely, the flash-to-flash period takes as long as one second in order to provide a short, intense pulse of illumination of tens of microseconds by inputting 15 joules of energy to the spectrophotometer. Accordingly, the spectrophotometer flashes only approximately four to ten times within a predetermined period of time during a spectral reflectance-measuring operation.